ARCHIVED REPORTS XR0002723 - Laserfiche WebLink

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1 5. 0 CONCLUSIONS AND RECOMMENDATIONS 5. 1 Vertical Extent of Soil Contamination ' The soil samples collected in this investigation confirm the presence of hydrocarbon contamination in the vicinity of MW-4 . Contamination has now been detected at a distance of approximately 1 10 feet north of the property line (beneath Harding Way) and also beneath the former pump island south of MW-4 (Figure 3) . North of MW-4 , maximum concentrations occur between 20 and 30 feet in B17. Elevated concentrations were also detected in the shallowest sample 1 (true depth 15 . 6 feet) . Low concentrations were detected in the deepest sample (depth 37 feet) . ' South of MW-4 , contamination extends downward to a depth of between 35 and 40 feet in B18. This agrees well with the lower limit of contamination in MW--4 itself. The presence of contaminated groundwater at a depth of approximately 47 feet suggests that soil contamination extends to greater depths somewhere off site. 5.2 Lateral Extent of Soil Contamination ' The eastern and western limits of soil contamination are defined by MW-5 and MW-1. Present data indicate that concentrations increase north of the site, but the northern limit cannot be determined with the available data. TPH-g concentrations are at least two orders of magnitude greater north of MW-4 than south of it. Total hydrocarbon concentrations in excess of 5000 ppm in sample B17-23 , as well as the presence of contamination at shallower depths (sample B17-18) lend support to WHF's previous suggestions that hydrocarbons migrated southward from a source located off-site to the north of MW-4 and mixed with a smaller contamination plume migrating northward from the dispenser island. In view of consistent groundwater flow to the northeast, it is also possible that the source of contamination lies off site to the west. It is our understanding that underground storage tanks that were formerly ' located on some of the surrounding properties have not been fully investigated; hence, much of the contamination evident in Figure 3 may not be the responsibility of the Campodonico-Tiscornia Trust. wThe samples collected from the floor and walls of the excavation during previous phases of investigation define the southern limit of contamination at shallow depth's (Figure 3) . This southern limit appears to be steeply inclined, and may lie just south of B18 at deeper depths (Figure 3) . 5. 3 Extent of Groundwater contamination Groundwater samples collected in this investigation indicate that 15